FIG. 1(a) through 1(c) show cross sectional views representing a method for forming a groove 16 in a semiconductor substrate 10 using a dry etching process. In FIG. 1(a), an oxide film 12 is formed on the surface of the semiconductor substrate 10, and a nitride film 14 is formed on the oxide film 12. In FIG. 1(b), a photoresist formed into a predetermined pattern is used to etch the nitride film 14 and the oxide film 12. In FIG. 1(c), the semiconductor substrate 10 is dry etched using the nitride film 14 as a mask. Then, the groove 16 is formed in the semiconductor substrate 10.
Since there is no marker or means present in the semiconductor substrate 10 which can tell whether the right depth has been reached for the groove 16, the dry etching for forming the groove 16 has to rely on the etching time only. However, the etching rate varies depending on the state within the chamber of the etching device, so this difference makes it difficult to form the groove 16 in a reliable fashion.
In another method of forming the groove 16, an oxygen ion is implanted into the semiconductor substrate 10 using the nitride film 14. The nitride film 14 is formed on the semiconductor substrate 10 in a predetermined pattern, and is used as the mask. The semiconductor substrate 10 is thermally oxidized by oxygen gas to form an oxide region defined by the nitride film 14. Then, the oxide region is eliminated by performing a wet etching to form the groove 16 in the semiconductor substrate 10.
However, the thermal oxidization of the semiconductor substrate 10 using the oxygen gas generates a relatively large portion of so called a bird's beak 19 as shown in FIG. 2. The bird's beak 19 denotes the portion of the oxide region 18 on the surface of the semiconductor substrate 10 masked with the nitride film 14. Accordingly, the deformation in the semiconductor substrate 10 makes it difficult to form a groove with a narrow width.